1. Field of the Invention
This invention relates to an endless belt for electrophotography, such as an intermediate transfer belt, a transfer material carrying belt or a photosensitive belt, a process for its production, and an image forming apparatus making use of it.
2. Related Background Art
Intermediate transfer belts, transfer material carrying belts and photosensitive belts are known as endless belts for electrophotography.
Compared with image forming apparatus in which images are transferred from a first image bearing member onto a second image bearing member (transfer material) fastened or attracted onto a transfer drum (e.g., Japanese Patent Application Laid-Open No. 63-301960), image forming apparatus making use of intermediate transfer belts have an advantage that a variety of second image bearing members can be selected without regard to their width and length, including thin paper (40 g/m2 paper) and up to thick paper (200 g/m2 paper) such as envelopes, post cards and labels. This is because any processing or control (e.g., the transfer material is held with a gripper, attracted, and made to have a curvature) is not required for the second image bearing member transfer material.
Image forming apparatus are also proposed which have a plurality of recording assemblies in which electrostatic latent images are formed on electrophotographic photosensitive members, the electrostatic latent images formed are developed and the developed images are transferred to a transfer material, where a full-color image is formed by transferring individual color toner images superimposingly to the transfer material while transporting it sequentially to the respective recording assemblies by means of a transfer material carrying belt.
It is also known to set up electrophotographic photosensitive members themselves in the form of endless belts for the purpose of achieving higher process speed or, especially in image forming apparatus having a plurality of developing assemblies and others, for the purpose of attaining the freedom in designing the arrangement of developing assemblies and others.
Image forming apparatus such as copying machines and printers making use of endless belts have various advantages as stated above. On the other hand, they also have some subjects for improvement.
For example, intermediate transfer belts are required to have a surface area not smaller than the image region, so that they are necessarily large in size and also required to have various properties such as resistance properties and surface properties, tending to result in a high production cost. They also have not necessarily a sufficient durability and tend to have to be frequently changed to new ones. As the result, this may raise the main-body price and running cost of copying machines and printers and also it may take more time and labor for their maintenance. In particular, because of market trends in recent years, it has increasingly become important to achieve lower prices and provide maintenance-free articles.
In addition, in order to form good color images, some problems must be solved which may occur because a plurality of colors are superimposed on the intermediate transfer belt.
One of them is a misregistration occurring between individual color images. In fine lines and characters, even a slight color misregistration tends to be conspicuous to provide a possibility of damaging image quality. The intermediate transfer belt is set across a plurality of shafts and is driven and rotated around them, where the tension applied to every part of the intermediate transfer belt is not necessarily uniform. Hence, the intermediate transfer belt may undergo a local elongation and, concurrent therewith, may cause a delicately uneven rotation. These are considered to come out as delicate color misregistration.
Another problem is occurrence of spots around line images.
A color image is formed by superimposing a plurality of toner images and hence has a larger quantity of toners per unit area than a monochromatic image. Especially in characters or letters and fine lines, toners are present in a large quantity on narrow lines. Moreover, individual color toners have electric charges with the same polarity and hence repulse one another electrostatically. Thus, they can be said to lie on the intermediate transfer belt in an unstable state.
Meanwhile, because of a difference in arcs drawn by the outer surface and inner surface of the intermediate transfer belt, produced when it passes the shafts over which it is set, the intermediate transfer belt elongates in the peripheral direction at its surface and in the vicinity thereof.
Thus, the toner images standing unstable and weak to external disturbance as stated above are disordered when the intermediate transfer belt passes the shafts, so that the spots around line images come to occur, as so considered.
Still another problem is half-tone image transfer performance. Faulty images tend to occur when the intermediate transfer belt has uneven resistance or uneven thickness.
In addition to these, the intermediate transfer belt always undergoes a tension and a repeated flexural elongation stress, and hence the intermediate transfer belt is required to have a material rigidity high enough to neither break nor crack even when used over a long period of time. The intermediate transfer belt made of resin also tends to cause what is called a creep, in which the above stress makes the belt elongate gradually with time in the peripheral direction. Any great change in size caused by the creep may make a difference from the original designing to aggravate color misregistration or may cause faulty images such as uneven halftone images. It may also cause a difficulty in the rotation of the intermediate transfer belt, acting as a great factor to shorten the life of the intermediate transfer belt.
For the achievement of cost reduction, which is another important subject, the intermediate transfer belt must be made thin-gage in order to reduce the quantity of materials constituting the belt, and also a production process having a smaller number of steps must be provided. Making the belt thin-gage also has the effect of less causing a transfer toner scatter and is an effective means, but on the other hand tends to cause a problem also in respect of durability.
Moreover, it is essential for the intermediate transfer belt to be provided, in its neighborhood, with a mechanism of applying a high voltage. Accordingly, as constituent materials therefor, high-safety materials are preferred that may fire or smoke with difficulty against any unforeseen accidents such as abnormal discharge and insulation failure.
However, satisfying all of these high image quality, high durability, low cost and safety involves technical difficulties. Accordingly, studies are made on intermediate transfer belts made of resin which satisfy these characteristics at a higher level.
As for the transfer material carrying belt, it is not the case that images are directly transferred onto the belt. However, in order to achieve a high image quality, the transfer material carrying belt is required to satisfy the same characteristics as those for the intermediate transfer belt, e.g., uniform resistance, surface properties, cost reduction, durability and safety. The same also applies to the photosensitive belt, on the surface of which images are directly formed.
Various processes for producing endless belts used in the intermediate transfer belts and so forth are already known in the art. For example, Japanese Patent Application Laid-Open No. 3-89357 and No. 5-345368 disclose a process for producing a semiconducting belt by extrusion. Japanese Patent Application Laid-Open No. 5-269849 also discloses a process in which a belt is obtained by joining both ends of a sheet to bring it into a cylindrical form. Japanese Patent Application Laid-Open No. 9-269674 discloses a process in which a belt is obtained by forming a multi-layer coating film on a cylindrical substrate and finally removing the substrate. Also, Japanese Patent Application Laid-Open No. 5-77252 discloses a seamless belt obtained by centrifugal molding.
However, e.g., in the extrusion, the production of a thin-layer belt which enables reduction of cost and prevention of spots around line images involves considerable difficulties when the die gap of an extrusion die is merely set in the same size as the desired belt thickness to carry out extrusion. Even if possible, such extrusion tends to cause uneven thickness and, as an effect thereof, uneven electrical resistance. In the case when both ends of a sheet are joined, the difference in height and decrease in tensile strength at the joint tend to come into question. Also, processes making use of solvents as in cast molding, the coating and centrifugal molding require many steps of preparing a coating solution, coating the solution and removing the solvent, resulting in a high cost.